1. Field of the Invention
The present invention relates to a nitride semiconductor device employing a group III nitride semiconductor and a method for producing the same.
2. Description of Related Art
Conventionally, the so-called CAVET (Current Aperture Vertical Electron Transistor) is known as a vertical transistor employing two-dimensional electron gas resulting from a semiconductor heterojunction as a channel such as an HEMT (High Electron Mobility Transistor).
FIG. 3 is a schematic sectional view of a conventional CAVET.
This CAVET includes a GaN (gallium nitride) substrate 101, a high-resistance layer (an n−-type GaN (gallium nitride) layer 102 in FIG. 3) having a low n-type impurity concentration stacked on a principal growth plane 101a of the GaN substrate 101 and a layer (an AlGaN (aluminum gallium nitride) layer 103 in FIG. 3, for example), having a different lattice constant from the n−-type GaN layer 102, laminated on the n−-type GaN layer 102, for example. In other words, the n−-type GaN layer 102 and the AlGaN layer 103 are successively epitaxially grown in order on the GaN substrate 101 with the interface parallel to the principal growth plane 101a. 
A p-type GaN layer 104 provided with an opening 105 on a position opposed to the AlGaN layer 103 is formed on an intermediate portion of the n−-type GaN layer 102 in the thickness direction.
In the vicinity of the interface between a portion located above the p-type GaN layer 104 and the AlGaN layer 103, two-dimensional electron gas 106 parallel to the principal growth plane 101a is formed in the n−-type GaN layer 102 due to the heterojunction therebetween.
A gate electrode 107 is formed on the AlGaN layer 103, to form a Schottky junction with the AlGaN layer 103. Two source electrode 108 are formed to be opposed to each other through the gate electrode 107, to form ohmic contact with the AlGaN layer 103 or the n−-type GaN layer 102.
A drain electrode 109 is formed on the back surface of the GaN substrate 101 opposite to the principal growth plane 101a, to form ohmic contact with the GaN substrate 101.
Thus, the source electrodes 108 and the drain electrode 109 are opposed to each other through the GaN substrate 101, to constitute the CAVET having a vertical structure.
When a bias voltage positive on the drain side is applied between the source and the drain in the CAVET having the aforementioned vertical structure, a potential difference is caused between the source and the drain, and a current flows from the drain electrode 109 to the source electrode 108. More specifically, when the bias voltage is applied between the source and the drain, the two-dimensional electron gas 106 moves due to the potential difference therebetween, and electrons reach the opening 105 of the p-type GaN layer 104.
The electrons reaching the opening 105 flow in the n−-type GaN layer 102 through the opening 105 to flow into the GaN substrate 101, due to the potential of the drain electrode 109. The electrons flowing into the GaN substrate 101 reach the drain electrode 109 through the GaN substrate 101.
Thus, the current flows from the drain electrode 109 to the source electrode 108, to allow the source and the drain to conduct.
However, the aforementioned CAVET capable of implementing the vertical structure has the so-called normally-on characteristic with a gate threshold voltage Vth of −16 V. In order to turn off the CAVET, therefore, a negative voltage must be applied to the gate electrode 107, to pinch off the two-dimensional electron gas 106.
Therefore, the principal growth plane 101a of the GaN substrate 101 may be defined by a nonpolar plane (an a-plane or an m-plane). When the principal growth plane 101a of the GaN substrate 101 is defined by a nonpolar plane, the two-dimensional electron gas 106 formed in the n−-type GaN layer 102 parallel to the nonpolar plane is utilized.
However, no remarkable polarization takes place on the nonpolar plane, and hence the two-dimensional electron gas 106 has a low electron density. This leads to such another disadvantage that the channel mobility of the CAVET is reduced, although the CAVET approaches the so-called normally-off characteristic with the gate threshold voltage Vth exhibiting a positive value.